Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. As used herein, pixels and sub-pixels are not distinguished and refer to a single light-emitting element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode (LED) displays.
Light emitting diodes (LEDs) incorporating in films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Typically, one of the electrodes is transparent and the other reflective. Light is emitted from a pixel when current passes through the light-emitting material and passes through the transparent electrode and out of the device. If the bottom electrode and substrate are transparent, the device is a bottom emitter. If the top electrode and cover are transparent, the device is a top emitter. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both. It is well known that light emitted from the high-index organic layers is trapped in the organic layers, the transparent electrode, and a transparent substrate (in a bottom-emitter configuration) due to the relatively high indices of refraction of those materials compared to air.
In an active-matrix device, active control elements are formed of thin films of semiconductor material, for example amorphous or poly-crystalline silicon, coated over the flat-panel substrate. Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and an electrode electrically connected in common. Control of the light-emitting elements is typically provided through a data signal line, a select signal line, a power connection and a ground connection.
One common, prior-art method of forming active-matrix control elements typically deposits in films of semiconductor materials, such as silicon, onto a glass substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials. In such active-matrix designs, each light-emitting element requires a separate connection to a driving circuit. The behavior of silicon transistors, made in either thin films or in crystalline silicon, changes in the presence of electromagnetic radiation, including visible light. Typically, exposing the thin-film transistors to light increases the carrier density in the transistors, causing more current to pass through the transistor. This, in turn, can increase the amount of current passed through light-emitting diodes, for example in organic light emitting diode displays. These changes in current cause non-uniformities in the display, increased or decreased brightness, or other unacceptable display behaviors.
This problem can be addressed in a thin-film transistor circuit by forming a light-shield on the substrate to shield transistors from light in an LCD, as disclosed in U.S. Pat. No. 6,525,341 issued Feb. 25, 2003 to Tsujimura, et al. A metal gate electrode is also disclosed that can serve to shield a transistor junction from light. U.S. Pat. No. 6,746,905 issued Jun. 8, 2004 to Fukuda discloses a light shield layer including an amorphous silicon carbide layer located below thin-film transistors. However, these structures can be limited to thin-film circuit designs on substrates. U.S. Pat. No. 6,636,284 issued Oct. 21, 2003 to Sato describes an electro-optical device with TFTs that includes an upper light shield layer and a lower light shield for an LCD. The upper light shield is formed in a grid-like configuration above the TFTs and can include a capacitor layer or conductive traces such as a data line or capacitive line. The lower light shield is formed on the substrate beneath the TFTs. This arrangement requires that conductive lines or a circuit capacitor be located directly above the TFTs, therefore limits the circuit layout, requires more processing layers than are desirable and might not be useful in other circuit designs and circuit manufacturing processes.
Employing an alternative control technique, Matsumura et al., in U.S. Patent Application No. 2006/0055864, describe crystalline silicon substrates used for driving LCD displays. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. However, there is no teaching of structures or methods for preventing light exposure to circuits in such a pixel-control device.
There is a need, therefore, for an improved structure for display devices employing LEDs and high-performance circuits that overcomes any problems with circuit performance changes in response to light exposure.